DC to DC power converters are required for many modern day applications. For example, such converters are required for high voltage, high power applications such as used in telecommunications, battery chargers, and uninterruptible power supplies with power factor correction.
As switching frequencies increase, the switching losses associated with the turn-on and turn-off of the devices also increase. In soft-switching DC to DC power converters, such as LLC resonant converter, the switching losses can be lower, allowing the resonant converter to operate at higher frequencies. Therefore, the use of resonant converters remains an interesting option for some applications requiring the previous specifications.
The output voltage of the LLC resonant converter is adjusted by changing the switching frequency. However, when the output voltage is far away from rated output voltage, for which the resonant tank's parameter is optimally designed, the performance will decrease. Therefore in practical, additional buck/boost is recommended to be used along with LLC resonant converter, which on the country results in, decreased power density and increased cost. To satisfy the requirement of output current ripple or reduced the output inductance, interleaved buck/boost is usually applied. However, this leads to a more complex control and drive system.
In order to overcome the problem, a conventional solution is to use multi-winding high frequency transformer to create multiple output voltage level while the LLC resonant converter can be designed in a small switching frequency range. An example is presented Francisco Canales et al., A High-Power-Density DC to DC Converter for High-Power Distributed Power Systems, Power Electronics Specialist Conference, 2003. PESC '03. 2003 IEEE 34th Annual. FIG. 4 of the paper shows a topology of rectifier used in a power DC to DC converter. Depending on the state of the power switches (S5, S6), three of the four taps of the secondary winding of the transformer are conducting for output AC power which in turn is converted into DC power at the output of the converter. One of the drawbacks that these prior art converters present is that it allows two-level DC voltage output at most. In order to raise the number of DC output voltage level, additional bidirectional blocking power switches have to be included. This in turn increases power loss, cost and complexity.